1. Field of the Invention
Fused glass multifocal lenses with particular reference to multifocal lenses having major portions formed of photochromic glass and glass segment portions of lower refractive index fused thereto.
2. Description of the Prior Art
Multifocal lenses having segment portions of high refractive index glass fused to major portions of relatively low refractive index glass are common in the art as evidenced by U.S. Pat. Nos. 2,755,706 and 2,958,162. Glasses used for the segment and major portions of such lenses, i.e., having compatible coefficients of expansion and softening temperatures, are readily available in colors or clear.
Since, however, these segment glasses do not provide a compatible expansion fit and do not have softening temperatures low enough to prevent impairment of the photo-response of known photochromic glass major portions after fusion, the manufacture of photosensitive multifocal lenses has been restricted either to those of one-piece construction of the type shown in U.S. Pat. Nos. 2,410,145 and 2,890,551, for example, or of the cemented type wherein the segments are cemented into countersinks of major portions of the lenses or the alternative of a photochromic overlay.
Cemented glass multifocal lenses are currently becoming obsolete in view of their not being able to withstand air-toughening temperatures or ion-exchange processing now used and/or contemplated for future use in the finishing of lenses for customer wearing purposes.
In the light of occupational safety and health regulations now requiring all glass lenses to be toughened either by air hardening or ion exchange process before wearing, glass photosensitive multifocal lenses capable of withstanding the required toughening processes have, heretofore, become extremely difficult if not impossible to produce. The cemented type tends to delaminate and/or otherwise become interfacially defective while, on the other hand, impairment of the required or desired photoresponse characteristics of photochromic lens major portions and interfacial defects result from attempts to fuse segment portions of conventional high refractive index glasses to the photochromic major portions.
A principle objective of the present invention is to provide a novel high refractive index multifocal lens segment glass having a coefficient of expansion which is compatible with currently available photochromic relatively low refractive index lens major portion glass, the high index segment glass having a softening temperature which is low enough to prevent impairment of photoresponse of the photochromic major portion glass during the operation of thermally sealing (i.e., fusing) the segment glass to the photochromic major portion glass.
Another object of the invention is to provide a family of high refractive index glasses suitable for thermal sealing to currently available low refractive index photochromic glasses without impairment of the photoresponse of the latter glasses and to provide improved multifocal lenses incorporating these high and low refractive index glasses.
The invention will be more readily understood from the following detailed description taken in conjunction with the accompanying drawings.